Sulfur recovery process

ABSTRACT

A method of recovering sulfur from a metal sulfate whereby a substantial part of the sulfur content is recovered as sulfur dioxide and the balance as hydrogen sulfide. The sulfate is contacted with reducing gases at high temperatures to produce a metal sulfide and a metal oxide under conditions favoring an offgas stream rich in sulfur dioxide; the metal sulfide may subsequently be converted to hydrogen sulfide and sulfur and sulfur compounds are recovered.

United States Patent [72] inventors Roy Edwin Camiabell; 3,401,0109/1968 Guerrieri 23/177 X Edwin Eddie Fisher, both of Midland, 2,740,6914/ 1956 Burwell 23/181 FOREIGN PATENTS [21] P 746643 514,993 11/1939England 231177 [22] F11ed July 22, 1968 643 479 9/1950 E ngland 23/177[45] Patented Sept. 21, 1971 t [73] Assignee Elcor Chemical CorporationOTHER REFERENCES Midland, Tex, Sisler et al., General Chemistry, pp. 34-35 (Macmillan 1949) Perry, Chem. Engrs. Handbook, sect. 9, pp. 58- 60,4th ed. (McGraw-l-lill 1963) [54] SULFUR RECOVERY PROCESS PrimaryExaminer-Oscar R. Vertiz 9 Claims, 3 Drawing Figs. AssistantExaminer-Charles B. Rodman 52 us. c1 23/224, Graves Raymnd 23/ 177 [51Int. Cl ..C0lb 17/44, ABSTRACT: A method of recovering Sulfur from 3meta] Colb 17/50 sulfate whereby a substantial part of the sulfurcontent is [50] Field of Search 23/224-226, recovered as lf dioxide andthe balance as hydrogen 137 fide. The sulfate is contacted with reducinggases at high tem- [56] References Cited peratures to produce a metalsulfide and a metal oxlde under cond1t1ons favonng an off-gas streamrrch 1n sulfur dloxlde, UNlTED STATES PATENTS the metal sulfide maysubsequently be converted to hydrogen 3,402,998 9/ 1968 Squires .123/181 sulfide and sulfur and sulfur compounds are recovered.

H o co H 16 2 2 Water Air Contact A Fm l sg g cg Sulfuric Acid um:Raductant 2:: 2 2 2 Process (cm) Preparation H O so -H so,

t a (cH H H 0 r-|S I co H 1 L H 50 L 2 H O co l9 1 CO i 1 Vent [0d J' CO+N Z Z l 1 ti Rock Reaction J 1 i I 20 Rock Dehydration I 804+ GasPreheat i szzarizri fiififfo c Crushed Gypsum Fired and/or -l- R ck -5 Qe h 0 2 z" lnus COSO 'ZHzO Waste Hoot I Pro heot a ggg f l ff- For Heat2 H 5 ype S CuSOp 2H O+A+ 1 4CD Absorption I g 8 in? 2 C0$O4+2H2O I Ca$O+4H: & Slurry I U 3 2 I AHL t CoS+4H O l I S Fucl L REAcToR------ l C0602 wmer Cc|5 Slurry A. C :(OH) Slurry Ball Mill Grind COS- Co(OH/ usPrepotion for Feed to C05 2! Column SULFUR RECOVERY PROESS Thisinvention relates to a process for recovering sulfur from a metalsulfate, and particularly, to a procedure by which a metal sulfate isreduced to provide separate streams of sulfur dioxide and hydrogensulfide.

Sulfur in its various forms is a very important and widely user.chemical. Such processes as synthetic fibers, rubber, plastics, paint,and oil refining depend heavily upon the availability of large amountsof sulfur, or sulfuric acid, at a relatively low cost. Approximately 26million long tons of sulfur were consumed in 1967 in the Free World andconsumption has been growing at the rate of seven to eight percentannually during the past five years.

World sulfur reserves from all sources are ample for the foreseeablefuture, but the present primary sources are limited in reserves andthese sources must be supplemented by other sources for future needs.Primary sources include those which now are producing most of thesulfur, such as Frasch mining, recovery from sour gas, and mineralproduction. Vast reserves of sulfur are contained in secondary sourcessuch as sulfide ores, oil, oil shales and tar sands, coal, gypsum, andanhydrite. Unfortunately, the processing technology has not beendeveloped to allow a significant amount of most of these secondarysources to supply sulfur at costs as low as the primary sources.

As competitive sources of sulfur, gypsum and anhydrite have been thesubjects of study for many years, but as yet there is no facility inthis country producing elemental sulfur from this source. There are afew plants recovering sulfur in the form of sulfuric acid in othercountries, but apparently the economics of these operations have notbeen favorable for this country.

One such process is employed in Europe to produce sulfuric acid fromanhydrite. In accordance with this process, a mixture of anhydrite,coke, and shale is heated to a sintering temperature in a rotary kiln.Sulfur dioxide is driven off and recovered for conversion into sulfuricacid. The residue or clinker is used in making Portland cement.

Elemental sulfur may be produced from gypsum by following the process ofFrench Pat. No. 375,469 published July 10, 1907. In accordance with theFrench Pat, calcium sulfate is first treated with reducing gases to formcalcium sulfide, carbon dioxide and water in accordance with thefollowing equa tions:

The calcium sulfide is then ground and combined with water to form aslurry. Carbon dioxide is collected from the first reaction, and thencontacted with the calcium sulfide slurry to cause a reaction inaccordance with the following equation:

The stream of hydrogen sulfide obtained as a product of the reaction isconverted to elemental sulfur in a Claus-type sulfur plant, and the CaCOis produced as a byproduct.

The process of the French patent requires 4.0 moles of reductant gases Hand CO in the ideal case to produce 1.0 mole of elemental sulfur. Thefollowing reactions indicate this relationship:

CaSO4+4H2 ,CaS+ 4H2O }4 moles of either reducing gas are required toform CaSO +4CO CaS+4CO one mole of Gas one mole of Gas forms one mole ofH 8, and then one mole of elemental sulfu Other reactions can occurduring the reduction reactions in which the same materials reacttogether to form different products. Both hydrogen and carbon monoxide,for example, can react with calcium sulfate to form calcium oxide andsulfur dioxide:

If these reactions can be made to occur, only one mole of reductant isrequired per mole of sulfur. This is one-fourth as much as is requiredwhen the sulfate is reduced to the sulfide. However, the requiredreactions for this to occur proceed at commercial rates only attemperatures above about 2,000 F., and the reactions are highlyendothermic. Thus, a commercial process would require the transfer ofsubstantial amounts of heat into the reacting materials, and thetransfer must take place at temperatures above 2,000 F. Supplying largequantities of heat at such a high temperature would require veryexpensive materials and an economic use of the large quantities of theresulting waste heat. These two factors have prevented any significantcommercial utilization of these reactions.

We have discovered that the required heat of reaction for the aboveendothermic reactions can be supplied by the exothermic heat of reactionresulting from the reduction of a portion of the calcium sulfate tocalcium sulfide. This internal generation of the required heat is highlyefficient, since there is essentially no heat lost, except for the minorheat loss through the wall of a commercial reactor. Supplying the heatby internal generation also has the advantage of allowing a very simplereactor vessel construction using mild steel with an internal lining ofcommonly available refractory material.

In addition, we have found that an improved process for producing sulfurfrom a metal sulfate is provided by reducing the sulfate underconditions which produce substantial quantities of sulfur dioxide in arich stream, as well as metal sulfide, and by thereafter contacting thesulfide with carbon dioxide in the presence of water to produce hydrogensulfide. The sulfur dioxide may be separately collected; if desired, itmay be used to manufacture sulfuric acid. The separate stream ofhydrogen sulfide may be converted to elemental sulfur in a conventionalmanner, as by a Claus-type reaction. Alternatively and in accordancewith a preferred embodiment of the invention, the S0 and H 8 may beproduced in stoichometric amounts and thereafter combined to provideelemental sulfur in a Claustype reaction.

The process described herein is of immediate commercial value because itwill permit economic recovery of sulfur from gypsum and anhydrite in theform of elemental sulfur and sulfuric acid, or in the forms of calciumsulfide, hydrogen sulfide, and sulfur dioxide. It has flexibility inthat the range of products can be varied according to local economicsituations without sacrificing the unique capability to recover sulfurat a cost lower than current technology and at a cost which iscompetitive in todays market.

Gypsum and. anhydrite are interchangeable for the processing describedsince the chemical reactions are identical. Differences in reactionrates and the dehydration heat requirements for these raw materials areso minor that proper design and operation will allow both materials tobe processed in the same equipment to make the same end products.

Hydrogen and carbon monoxide are commonly used reducing gases; formulaedescribing their reactions in the process of this invention are given inthe following table to illustrate the concept of balancing the heats ofreaction:

1 Released or required per mole of calcium sulfate calculated at 2,100F.,

expressed as kcaL/g. mole. 7 V

Small amounts of sulfur also emerge from the reaction in such otherforms as hydrogen sulfide, carbonyl sulfide, and

elemental sulfur, depending upon the specific conditions maintained inthe reactor.

For a further understanding of the invention, reference is made to theaccompanying drawings, wherein:

FIG. 1 is a chart illustrating one variable condition of the process ofthis invention,

FIG. 2 is a flow diagram of the process of the present invention, and

FIG. 3 is another flow diagram of one embodiment of the presentinvention.

FIG. 1 of the drawings illustrates the effect of balancing theendothermic and exothermic heats of reactions as the ratio of hydrogento carbon monoxide is varied. This curve is calculated for an averagereaction zone temperature of 2,lO F.,

neglecting heat losses, variation of temperature throughout a practicalreaction zone with a finite depth, minor side reactions, incompleteconversion of the gas and solid, etc. FIG. 1 represents the maximumpercentage of sulfur which it is possible to produce as sulfur dioxideunder ideal conditions and is based on commonly accepted thermodynamicdata from the chemical literature.

By following hydrogen through the reactions outlined in Table I, it willbe noted that the heat released by hydrogen in the reaction forming CaSis considerably less than the heat required by hydrogen in forming SO,Carbon monoxide, on the other hand, has a more favorable heat balancebetween the two reactions. Carbon monoxide is thus a more desirablereductant than hydrogen in processes where it is desirable to producethe maximum amount of sulfur dioxide. Thus, where desired for aparticular installation, the performance may be improved by adjustingthe composition of the reducing gas. Such adjustment can be effected forinstance, by introducing additional carbon dioxide which will react atelevated temperatures with hydrogen to form carbon monoxide and watervapor. This reaction is endothermic and heat must be supplied to effectthe conversion to carbon monoxide and increase the carbon monoxide tohydrogen ratio. This adjustment of concentration may be performed in aseparate processing step between the reduction gas preparation and thereactor. The technology of preparing various reducing gas streams bysuch means as partial oxidation, steam reforming etc. is well known.

Metal sulfates which may be treated according to the process of thepresent invention include aluminum sulfate, barium sulfate, cadmiumsulfate, calcium sulfate, cerium sulfate, chromium sulfate, cobaltsulfate, potassium sulfate, magnesium sulfate, lithium sulfate, sodiumsulfate, nickel sulfate, lead sulfate, strontium sulfate, thoriumsulfate, titanium sulfate, vanadium sulfate, and zinc sulfate.

The invention is particularly useful for reducing alkaline earth metalsulfates such as calcium sulfate, magnesium sulfate, barium sulfate, andstrontium sulfate. Calcium sulfate, in the form of gypsum or anhydrite,is of the most immediate commercial interest. However, it is recognizedthat the process disclosed herein, with suitable variations inconditions, is applicable to the other metal sulfates listed above.

Another means of favoring the additional reactions that increase theamount of sulfur dioxide produced while maintaining other conditions ofthe reduction reaction constant is by supplying additional heat to thereaction bed. Particulate inert solids heated to temperatures in excessof that existing in the reaction bed may be fed to the bed continuouslywith the sulfate and reducing gases. Examples of suitable solids areceramic materials which remain solid at the high temperatures involved.Another means of introducing heat into the reaction zone is through theuse ol'electric heaters located in the reaction bed. Still anotherprocedure is the recycling of preheated gases which transfer theirsensible heat to the reaction bed.

A still further alternative means is the addition or recycling of a gaswhich will react in the reaction bed or with the product rock leavingthe reactor and liberate itsexothermic heat of reaction. The followingequations give illustrative reactions:

Approximate heat 1 -17 exothermic.

1 Released or required per mole of calcium calculated at 2,l00 F.,kcaL/g. mole. H

Approximate heat 1 -108 exothermic.

Relcased or required per mole of calcium calculated at 2,100" F.,kcaL/g. mole.

These reactions release large amounts of exothermic heat of reactionwithin the reaction bed.

In accordance with one preferred embodiment of the process of thisinvention, the process conditions and concen trations of the reducinggas are adjusted to obtain exactly onethird of the sulfur in the form ofsulfur dioxide. The remaining two-thirds of the sulfur in the calciumsulfate is then converted to hydrogen sulfide, for example, as describedin French Pat. No. 375,469. The sulfur dioxide gas stream is then mixedwith the hydrogen sulfide gas stream and converted to elemental sulfurin a Claus-type sulfur plant. The net result is a savings ofapproximately 25 percent of the reductant gas usage resulting from theprocess of converting all the sulfur to hydrogen sulfide as in FrenchPat. 375,469. Since the cost of the reductants is a major cost in theproduction of sulfur, this provides a substantial savings.

Where the composition of the reducing gas permits, we prefer to producethe gases in a hydrogen sulfide-to-sulfur dioxide ratio of two to onefor the most efficient production of elemental sulfur. If theconcentration of the reducing gas would not permit the desired ratio ofhydrogen sulfide to carbon monoxide, the cost of adjusting the reducinggas concentration would, of course, be compared to the increased use ofreductants in order to achieve this ratio for sulfur recovery. If theratio is lower than this, the excess sulfur in the form of sulfurdioxide can be recovered for other uses, such as production of sulfuricacid. The operating conditions of the reactor can be varied to changethe percentage of sulfur produced as sulfur dioxide. For example,lowering the temperature of the solid or gas feed streams will lower theamount of sulfur dioxide produced. If the ratio is higher than two toone, air or oxygen is mixed and reacted with the excess hydrogen sulfidepresent before it enters the sulfur plant. This oxygen-containing streamis controlled to adjust the ratio to two to one. Any sulfur recovered assulfur dioxide represents a savings in the use of reductant gases, evenwhen the amount is more or less than one-third of the total sulfur.

A second preferred method of using the process of this invention is toutilize the most readily available source of reducing gases. In somesections of the country, such a source is from the steam reforming ofhydrocarbons, especially the steam reforming of natural gas. A stream ofreducing gas from the reforming of natural gas is capable of producingsulfur dioxide in quantities approaching one-third of the total sulfurwith proper gas quality and process design. This preferred system hasthe advantages of proven processes for the reduction gas preparation,convenience, and having sufficient carbon in the reducing gas to furnishmost or all of the carbon dioxide necessary for converting the calciumsulfide to hydrogen sulfide. The ratio of carbon to hydrogen in thehydrocarbon is a factor in the economics. A minimum amount of carbon isrequired to generate the necessary carbon dioxide in the reactor bed forthe subsequent release of hydrogen sulenriched by the addition of carbonmonoxide to provide a stream of reducing gases containing approximatelyequal volumes of carbon monoxide, and hydrogen, with carbon dioxide andwater being present in equilibrium amounts and with trace quantities ofnitrogen. The exact amount of carbon monoxide required will depend uponthe exact operating conditions and design of equipment, but iscontrolled to maintain a hydrogen sulfide-to-sulfur dioxide ratio ofexactly two-toone. The steam reforming step is shown in the attacheddrawing as Block 10. Supplemental amounts of carbon monoxide areprovided through line a as required.

Gypsum ore, preferably crushed and screened to a particle size range ofabout V4 to one and we inches in maximum dimensions, is dehydrated asshown at block II in FIG. 2. Anhydrite,

which is interchangeable, may be substituted for the gypsum, in whichcase a dehydration step is unnecessary. Following the dehydration step,the rock is introduced into a reaction vessel, preferably a shaftfurnace, which is represented in the diagram of FIG. 2 by Blocks I2, I3and 14. Block 12 represents the top section of the shaft furnace wherethe sulfate is preheated utilizing the hot gases produced at a laterstage in the process. As the rock slowly descends through the furnace,it is gradually heated from its temperature at the time of introductionto the temperature of the reaction zone. This is accomplished by theexchange of heat from the hot stream of gaseous products flowing upthrough the descending rock.

Block 13 represents the reaction zone of the reactor. The temperature atthis stage may range from 2,000 F. to 2,500 F. and the calcium sulfateattains this temperature during its downward passage through thereactor. A series of reactions take place in the reaction zone asindicated in Block 13. The reduction of the calcium sulfate to calciumsulfide by reaction with carbon monoxide and hydrogen are shown in thelower two equations. The first two equations represent the reactionswhich produce sulfur dioxide. The resulting calcium sulfide and calciumoxide continue their passage downwardly through the column and arecooled and quenched as shown in block 14. The sulfur dioxide and carbondioxide as well as any water vapor produced during the reaction passupwardly through the freshly introduced calcium sulfate and perform apreheating function and are thereafter recovered through Line 15.

The sulfur dioxide stream may then be used to produce sulfuric acid inaccordance with the equations shown in Block 16. In that event, thecarbon dioxide is recovered and carried through Line 17 to the calciumsulfide reactor represented by Block 18. Optionally, the sulfur dioxidestream is carried directly by Line 19 to be combined with the hydrogensulfide produced by reactor 18, to produce elemental sulfur in a unitrepresented by Block 20.

The calcium oxide, which is converted to calcium hydroxide during thequenching step, and the calcium sulfide from the cooling step at BlockI4, are carried to a ball mill and ground in the step represented byBlock 21. This mixture is fed to a calcium sulfide reactor representedby Block 18, where the calcium sulfide is converted to hydrogen sulfide.The calcium hydroxide is converted to calcium carbonate during thisstep. Essentially, all calcium is removed as calcium carbonate as thefinal product of the reaction. Minor processing steps such as one-thirdto sulfur dioxide.

dust removal and heat exchange are not shown on FIG. 2 for EXAMPLE IFIG. 3 is a flow diagram describing an example of the reduction ofgypsum according to this invention. In accordance with this example,about one-third of the sulfate sulfur is converted to S0 and the balanceto H 8. The numerals which appear in boxes in FIG. 3 represent amaterial balance and give the amount of the indicated reactant at thatpoint in the process in pound moles, for each pound moles of gypsum fedto the process.

Crushed gypsum ore, having particle sizes within the approximately rangeof 541 inch to 1-5: inch, is introduced as steam 30. The gypsum is thencompletely dehydrated by heating according to the following overallfonnula:

After dehydration, the dehydrated calcium sulfate is fed into the top ofthe shaft furnace reactor 31. The reactor is one which can processapproximately 7,500 tons per day of gypsum with a feed size range ofinch to l-% inch, and has a reaction bed which is approximately 20 feetID by 60 feet high. The reactor is operated at 100 p.s.i.g. pressure,with a pressure drop of 10 to 15 p.s.i. through the reactor bed.

The CaSO is fed into the top of the shaft furnace 31, and is preheatedas it moves downward by gravity through the upflow of reducing gases.

A reducing gas containing approximately equal volumes of CO and H,, aswell as an equilibrium amount of CO and H 0 is prepared and isintroduced into the bottom of the reactor as stream 32 at a temperatureof about 1 ,800 F.

Assuming approximately 96 percent conversion of the (32180 and thereducing gases, approximately I50 pound moles of H and ISO pounds molesof C0 are required for each 100 pound moles of CaSO The CaSO, is reducedin the reaction zone according to the following equations.

The maximum temperature reached in the reaction zone is approximately2,200 F.

Approximately one-third of the sulfur in the CaSO. feed leaves with thegas stream 33 in the form of S0 while approximately two-thirds of thesulfur emerges in the rock product as CaS, along with a small amount ofunconverted CaSO The calcium sulfate dust particles produced duringdownward passage of the rock are removed from stream 33 downstream ofthe reactor.

The reactor rock product, stream 34, is quenched in water, ground to afine slurry and carried to a gas-liquid reactor 35 where it is reactedwith CO supplied by gas stream 36 recovered from a later stage in theprocess.

Reactor 35 is a tall, liquid-filled column in which the CO containinggas stream 36 is bubbled up through the descending CaS slurry.Sufficient carbon is supplied to the process to cause all convertedcalcium to leave as calcium carbonate. If there is a carbon deficiency,it may be supplied from an outside source. For example, flue gasescontaining carbon dioxide may be compressed and added to gas stream 36to balance the requirements. No deficiency exists in the example beingdiscussed. The overall reaction in the Gas reactor is exothermic. Thereaction rate and temperature attained is dependent on the percentsolids in the slurry; a slurry concentration of about 15 percent isused. The CaO, which has reacted with water during quenching andgrinding to become Ca(OH) in the slurry, and the CaS, are converted to H8 and CaCO The CaCO -water slurry leaves the bottom of the reactorvessel 35 as stream 37 for disposal, water reclaiming or utilization inadditional processing. Because the reactor is operatedwith a smallexcess of CO in the gas stream 36 over that required for a completeconversion of the Cats to H 8, stream 37 contains only small amounts ofunreacted CaS.

The gas leaving the top of the reactor 35, stream 38, containsessentially all of the sulfur which entered as CaS, in the form of H Sgas. Excess CO recycled inert gases, and H vapor are also present. Thegas stream 33 leaving the top of the shaft furnace reactor 31, containsprimarily CO S0 and H 0, and small amounts of unconverted H and smallamounts of unconverted CO. The ratio of sulfur produced as H 8 to sulfurproduced as S0 is controlled at about 2:1. The gas stream 33, after dustcollection and dehydration, is mixed with gas stream 38 and reactedaccording to the following:

2H S+SO 3S+2H O This reaction is accomplished in the same manner as in aconventional Claus-process sulfur plant, which is weli known in theindustry. The gas stream is passed down through a bed of catalyst, suchas bauxite, at temperatures above the sulfur dew point. Sulfur iscondensed downstream of the catalyst bed by cooling the gas. The gasstream is then reheated and passed through a second catalyst bed and asecond sulfur condenser. The resulting gas stream 39 contains only aminor amount of unrecovered sulfur, but a large amount of carbon dioxideand water vapor. A portion of this stream is returned to react withcalcium sulfide in column 35 through stream 36. Excess CO and inertgases are vented as stream 39 to the atmosphere. The small amounts ofunrecovered sulfur in all forms is shown as elemental sulfur, and thehydrogen and carbon monoxide are shown as remaining unchanged forsimplicity in the material balance on FIG. 3. All these components areactually in various equilibrium forms. High purity molten sulfur, stream7 40, is drained to storage.

We claim:

1. A process for the gaseous reduction of calcium sulfate comprising thestep of heating the sulfate to a temperature within the approximaterange of 2,000" P. to 2,500 F. while contacting the sulfate withreducing gases containing hydrogen and carbon monoxide to concurrentlyproduce calcium sulfide, calcium oxide, and substantial quantities ofsulfur dioxide.

2. A process for the gaseous reduction of calcium sulfate comprising thesteps of heating the sulfate to reaction temperature by transferringheat from hot gaseous products of a subsequent reduction reaction of thesulfate with reducing gases, before the sulfate enters the reactionzone, reacting the sulfate with reducing gases containing hydrogen andcarbon monoxide at a temperature with the approximate range of 2,000 F.to 2,500 F. to produce calcium sulfide, calcium oxide, and substantialquantities of sulfur dioxide, and recovering the products. n V

3. A process for the gaseous reduction of calcium sulfate comprising thesteps of heating the sulfate to reaction temperature by transferringheat from hot gaseous products of a elasaawusafisastthsai fa wth.rssisdnasaseshsfas the sulfate enters the reaction zone, reacting thesulfate with reducing gases containing hydrogen and carbon monoxide at atemperature with the approximate range of 2,000 F. to 2,5000 F. toproduce calcium sulfide, calcium oxide, and substantial quantities ofsulfur dioxide. thereby converting from about five percent to aboutpercent of the sulfur to sulfur dioxide, and recovering the products.

4. A process for the gaseous reduction of calcium sulfate comprising thesteps of heating the sulfate to reaction temperature by transferringheat from hot gaseous products of a subsequent reaction of the sulfatewith reducing gases before the sulfate enters the reaction zone, thesulfate is reacted at a temperature within the approximate range of a2,000 F. to 2,500 F. with reducing gases containing hydrogen and carbonmonoxide in a molar ratio which converts approximately onethird of thesulfate sulfur to sulfur dioxide, and concurrently converting theremaining two-thirds of the sulfate to calcium 5. Process of claim 4,wherein the calcium sulfide is reacted with carbon dioxide and water toform hydrogen sulfide, and the sulfur dioxide and hydrogen sulfide arecombined to form elemental sulfur.

6. A process for recovering elemental sulfur from gypsum which comprisesa. crushing and screening-the gypsum to a uniform particle size,

b. dehydrating the gypsum to drive off its water of crystallization,

c. heating the calcium sulfate up to reaction temperatures by exchangingheat with the hot gaseous products of the subsequent reaction, occurringin step (d) contacting the calcium sulfate with a stream of reducinggases comprising hydrogen and carbon monoxide in approximately equalquantities at a temperature within the approximate range of 2,000 F. to2,5000 F. to convert one-third of the sulfur in the metal sulfate tosulfur dioxide,

e. collecting the sulfur dioxide produced during the reduction reaction.I

f. converting the calcium sulfide to hydrogen sulfide, and

g. combining the sulfur dioxide and hydrogen sulfide streams to produceelemental sulfur.

7. The process of claim 6, wherein less than one-third of the sulfurcontained in the metal sulfate is converted to sulfur dioxide and aportion of the hydrogen sulfide is converted to sulfur dioxide tocontrol the ratio of hydrogen sulfide-to-sulfur dioxide ratio at 2-to-lfor the production of elemental sulfur.

8. The process of claim 4, wherein the calcium sulfide is reacted withcarbon dioxide and water to form hydrogen sulfide, and wherein all orpart of the sulfur dioxide is converted to sulfuric acid and thehydrogen sulfide is converted to elemental sulfur or sulfuric acid.

9. The process of claim 8, wherein carbon dioxide from flue gases or anoutside source is added as needed to provide sufficient carbon dioxideto convert all of the calcium to calcium carbonate.

2% UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent3,607,068 Dated September 21, 1971 Inventor(s) Roy Edwin Campbell andEdwin Eddie Fisher It is certified that error appears in theabove-identified patent and that: said Letters Patent are herebycorrected as shown below:

1-- First page, address of the inventors, Midland, Mich. .1

should be Midland, Tex.--,-

Column 5 line 25, block should be -Block--; line #9, block" should be--Block-,-

Column 6, lines 15 and 16, approxirnately should be approximate-5 line17, steam" should be -stream--;

1.351332, 4-1/2 inch should be +l/ r inch-3 line 36, "pounds" should be-pound-- line rh should be H HC&(OH)22 should be -Ca(OH) --5 Column 7line 15, after "S0 insert line 42, With" should be wi.thin- Column 8line 33, with should be ---within,'

line M, "2.3500031 should 2,500l-.-;

line after sulfur (first occurrence) insert --in the sulfate--;

line l3, after of delete "a"; and

line 35, 2,5000F.* should he --2,50oF.--.

Signed and sealed this 18th day of April 1972.

LLsEAL) J EDWARD MJ LETCBERJR. ROBERT GOTTSCHALK Attesting OfficerCommissioner of Patents

2. A process for the gaseous reduction of calcium sulfate comprising thesteps of heating the sulfate to reaction temperature by transferringheat from hot gaseous products of a subsequent reduction reaction of thesulfate with reducing gases, before the sulfate enters the reactionzone, reacting the sulfate with reducing gases containing hydrogen andcarbon monoxide at a temperature with the approximate range of 2,000* F.to 2,500* F. to produce calcium sulfide, calcium oxide, and substantialquantities of sulfur dioxide, and recovering the products.
 3. A processfor the gaseous reduction of calcium sulfate comprising the steps ofheating the sulfate to reaction temperature by transferring heat fromhot gaseous products of a subsequent reaction of the sulfate withreducing gases before the sulfate enters the reaction zone, reacting thesulfate with reducing gases containing hydrogen and carbon monoxide at atemperature with the approximate range of 2,000* F. to 2,5000* F. toproduce calcium sulfide, calcium oxide, and substantial quantities ofsulfur dioxide. thereby converting from about five percent to about 95percent of the sulfur to sulfur dioxide, and recovering the products. 4.A process for the gaseous reduction of calcium sulfate comprising thesteps of heating the sulfate to reaction temperature by transferringheat from hot gaseous products of a subsequent reaction of the sulfatewith reducing gases before the sulfate enters the reaction zone, thesulfate is reacted at a temperature within the approximate range of a2,000* F. to 2, 500* F. with reducing gases containing hydrogen andcarbon monoxide in a molar ratio which converts approximately one-thirdof the sulfate sulfur to sulfur dioxide, and concurrently converting theremaining two-thirds of the sulfate to calcium sulfide.
 5. Process ofclaim 4, wherein the calcium sulfide is reacted with carbon dioxide andwater to form hydrogen sulfide, and the sulfur dioxide and hydrogensulfide are combined to form elemental sulfur.
 6. A process forrecovering elemental sulfur from gypsum which comprises a. crushing andscreening the gypsum to a uniform particle size, b. dehydrating thegypsum to drive off its water of crystallization, c. heating the calciumsulfate up to reaction temperatures by exchanging heat with the hotgaseous products of the subsequent reaction, occurring in step (d) d.contacting the calcium sulfate with a stream of reducing gasescomprising hydrogen and carbon monoxide in approximately equalquantities at a temperature within the approximate range of 2,000* F. to2,5000* F. to convert one-third of the sulfur in the metal sulfate tosulfur dioxide, e. collecting the sulfur dioxide produced during thereduction reaction. f. converting the calcium sulfide to hydrogensulfide, and g. combining the sulfur dioxide and hydrogen sulfidestreams to produce elemental sulfur.
 7. The process of claim 6, whereinless than one-third of the sulfur contained in the metal sulfate isconverted to sulfur dioxide and a portion of the hydrogen sulfide isconverted to sulfur dioxide to control the ratio of hydrogensulfide-to-sulfur dioxide ratio at 2-to-1 for the production ofelemental sulfur.
 8. The process of claim 4, wherein the calcium sulfideis reacted with carbon dioxide and water to form hydrogen sulfide, andwherein all or part of the sulfur dioxide is converted to sulfuric acidand the hydrogen sulfide is converted to elemental sulfur or sulfuricacid.
 9. The process of claim 8, wherein carbon dioxide from flue gasesor an outside source is added as needed to provide sufficient carbondioxide to convert all of the calcium to calcium carbonate.